Two-step material decomposition calibration method for a full size photon counting computed tomography system

ABSTRACT

A method and a system for providing calibration for a photon counting detector forward model for material decomposition. The flux independent weighted bin response function is estimated using the expectation maximization method, and then used to estimate the pileup correction terms at each tube voltage setting for each detector pixel.

BACKGROUND Technical Field

The disclosure relates to material decomposition in a full size photoncounting computed tomography system.

Description of the Related Art

Computed tomography (CT) systems and methods are typically used formedical imaging and diagnosis. CT systems generally create projectionimages through a subject's body at a series of projection angles. Aradiation source, such as an X-ray tube, irradiates the body of asubject and projection images are generated at different angles. Imagesof the subject's body can be reconstructed from the projection images.

Conventionally, energy-integrating detectors (EIDs) and/orphoton-counting detectors (PCDs) have been used to measure CT projectiondata. PCDs offer many advantages including their capacity for performingspectral CT, wherein the PCDs resolve the counts of incident X-rays intospectral components referred to as energy bins, such that collectivelythe energy bins span the energy spectrum of the X-ray beam. Unlikenon-spectral CT, spectral CT generates information due to differentmaterials exhibiting different X-ray attenuation as a function of theX-ray energy. These differences enable a decomposition of the spectrallyresolved projection data into different material components, forexample, the two material components of the material decomposition canbe bone and water.

Even though PCDs have fast response times, at high X-ray flux ratesindicative of clinical X-ray imaging, multiple X-ray detection events ona single detector may occur within the detector's time response, aphenomenon called pileup. Left uncorrected, pileup effect distorts thePCD energy response and can degrade reconstructed images from PCDs. Whenthese effects are corrected, spectral CT has many advantages overconventional CT. Many clinical applications can benefit from spectral CTtechnology, including improved material differentiation since spectralCT extracts complete tissue characterization information from an imagedobject.

One challenge for more effectively using semiconductor-based PCDs forspectral CT is performing the material decomposition of the projectiondata in a robust and efficient manner. For example, correction of pileupin the detection process can be imperfect, and these imperfectionsdegrade the material components resulting from the materialdecomposition.

In a photon counting CT system, the semiconductor-based detector usingdirect conversion is designed to resolve the energy of the individualincoming photons and generate measurement of multiple energy bin countsfor each integration period. However, due to the detection physics insuch semiconductor materials (e.g. CdTe/CZT), the detector energyresponse is largely degraded/distorted by charge sharing, k-escape, andscattering effects in the energy deposition and charge inductionprocess, as well as electronic noise in the associated front-endelectronics. Due to finite signal induction time, at high count-rateconditions, pulse pile-up also distorts the energy response, asdiscussed above.

Due to sensor material non-uniformity and complexity of the integrateddetection system, it is impossible to do accurate modeling of suchdetector response for a PCD just based on physics theories or MonteCarlo simulations with a certain modeling of the signal inductionprocess, which modeling determines the accuracy of the forward model ofeach measurement. Also, due to uncertainties in the incident X-ray tubespectrum modeling, the modelling introduces additional errors in theforward model, and all these factors eventually degrade the materialdecomposition accuracy from the PCD measurements, therefore thegenerated spectral images.

Calibration methods have been proposed to solve similar problems inliterature. The general idea is to use multiple transmissionmeasurements of various known path lengths to modify the forward modelsuch that it agrees with the calibration measurements. Some ideas areapplied on estimation of the X-ray spectrum in conventional CT, seeSidky et al., Journal of Applied Physics 97(12), 124701 (2005) and Duanet al., Medical Physics 38(2), February, 2011, and later adopted onphoton-counting detectors to estimate the combined system spectralresponse, see Dickmann et al., Proc. SPIE 10573, Medical Imaging 2018:Physics of Medical Imaging, 1057311 (Mar. 9, 2018). However, there canbe many variations in the detail design and implementation of thecalibration method, especially considering the application feasibilityin a full 3rd generation CT geometry, which has not been demonstrated ordocumented in literature so far.

SUMMARY

The embodiments presented herein relate to a two-step calibration methodfor the PCD forward model for material decomposition. The methodconsists of two parts: 1) estimation of the flux independent weightedbin response function S_(wb) (E) using the expectation maximization (EM)method, and 2) estimation of the pileup correction term P_(b)(E, N_(b),N_(tot)). Once S_(wb) (E) is estimated from the calibration at each tubevoltage (kVp) setting for each detector pixel, it is saved as a softwarecalibration table in the system. It is then used as an input to estimatethe pileup correction terms P_(b)(E, N_(b), N_(tot)) at higher fluxscans. Both tables are then used for the material decomposition inoperational scans to estimate the basis material path lengths. Thecalibration tables are updated from time to time based on thesystem/detector performance variations.

BRIEF DESCRIPTION OF THE DRAWINGS

The application will be better understood in light of the descriptionwhich is given in a non-limiting manner, accompanied by the attacheddrawings in which:

FIG. 1 shows an example of a PCD bin response function S_(b)(E) for aphoton counting detector. Each curve stands for an example function foreach energy bin.

FIG. 2 shows a material decomposition calibration and processingworkflow.

FIG. 3 shows normalized linear attenuation coefficients for differentmaterials.

FIG. 4 shows a schematic of a calibration structure design, where thepileup correction tables P_(b) are generated and used for each mAindividually.

FIG. 5 shows a schematic of another calibration structure design, wherea universal pileup correction table P_(b) is generated for the entire mArange.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, material, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the application, but do not denote thatthey are present in every embodiment.

Thus, the appearances of the phrases “in one embodiment” or “in anembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the application.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

In a transmission measurement using a photon counting energy-resolvingdetector (PCD), the forward model can be formulated as below:N _(b)(_(1, . . . ,))=N ₀ ×∫dE w(E)S _(b)(E)exp(−Σμ_(m) l _(m)),  (1)where S_(b)(E) denotes the bin response function defined asS_(b)(E)=∫_(EbL) ^(EbH)R(E, E′)dE′ where R(E,E) is the detector responsefunction, and E_(bL), and E_(bH) are the low and high energy thresholdof each counting bin. FIG. 1 shows an example model of a typical S_(b)(E) function for a PCD, where a long tail above the energy window isinduced by charge sharing, k-escape and scattering effect. The lowenergy tail is mostly due to the finite energy resolution from theassociated electronic noise. N₀ is the total flux from an air scan,μ_(m) and l_(m) are the m^(t) ^(h) basis material linear attenuationcoefficient and path length. w(E) is the normalized incident X-rayspectrum. In practice, both w(E) and S_(b) (E) are not exactly known,and they can be combined as one term, S_(wb) (E)=w(E)S_(b) (E), calledthereafter the weighted bin response function. If S_(wb) (E) can becalibrated through measurements, the decomposition problem at low fluxcondition can be well solved.

For a high flux scan condition (e.g. a few percent of pulse pileup),pulse pileup introduces additional spectral distortion in themeasurement. One way to correct for the pileup effect is to introduceadditional correction terms (e.g. see Dickmann above, who uses themeasured count rate(s) as input). And this type of additionalcalibration is based on an accurate estimation of the flux independentweighted bin response S_(wb) (E). How to estimate S_(wb) (E) in a fullsize 3rd generation CT system is a first problem solved in the presentapplication.

At typical CT clinical scan conditions, it is common to encounter a fewpercent or higher pulse pileup for some measurements. The resultingeffect in material decomposition depends on the measured spectrum aswell as the flux. Without knowing the actual detector response, one canonly do a limited number of transmission measurements to adjust theforward model. For a full CT system in clinical setting, it is crucialto have a feasible calibration procedure. Therefore, how to efficientlyparameterize the model and optimize the calibration procedure is thesecond problem solved in the present application.

Additional practical challenges of conducting such a two-stepcalibration in a full size CT system include the following, and theyhave not been addressed or solved in prior arts, which are mostlytargeted for small scale benchtop systems: fan-angle dependent weightedspectral response due to beam pre-filtration (e.g. bowtie filters);minimum flux limited due to the X-ray tube operational specificationsand fixed system geometry; full detector ring calibration with variousdetector response across the pixels; limited space for in-systemcalibration phantom positioning; complication when calibrating on arotating system with anti-scatter-grids; calibration systematic errorand related mechanical design tolerances; non-ideal detectors withuniformity issue on energy resolution, counting, and drifts of theenergy threshold settings, etc.

All the above non-ideal factors need to be considered for a photoncounting CT to reach image qualities competitive to conventional energyintegrating detector (EID)-based systems which have much simplerdetector response modeling and related calibrations, while maintaining asimilar calibration procedure/workflow that does not significantlyincrease the system down time.

In one non-limiting embodiment, a two-step calibration method for thePCD forward model for material decomposition is applied. The methodconsists of two parts: 1) estimation of the flux independent weightedbin response function S_(wb) (E) using the expectation maximization (EM)method, and 2) estimation of the pileup correction term P_(b)(E, N_(b),N_(tot)) which is a function of energy (E) and the measured bin counts(N_(b), N_(tot)), where N_(b) is the individual bin count and N_(tot) isthe total count of all the energy bins. The calibrated forward model canbe expressed as:N _(b)(l _(1, . . . ,M))=N ₀∫^(Emax) dES _(wb)(E)*P _(b)(E,N _(b) ,N_(tot))exp(−Σμ_(m) l _(m))  (2)

Here, instead of using only two materials, as in prior arts (e.g., seeDickmann), the method uses 2-5 different materials such aspolypropylene, water, aluminium, titanium/copper, and k-edge materialsto calibrate the weighted bin response function S_(wb)(E) at low flux.With more selective materials used in the calibration, the number oftotal path lengths is reduced to achieve equivalent or better results.

Step 1: With an appropriate tube spectrum modelling to capture thecharacteristic peaks in the incident spectrum, and a physical model tosimulate the photon-counting detector spectral response, an initialguess of S_(wb) (E) can be produced. By using the EM method (e.g., seeSidky), S_(wb) (E) can be reliably estimated for this veryill-conditioned problem based on a few transmission measurements.

Here, P_(b)(E, N_(b), N_(tot)) is assumed to be constant in Step 1. Thecalibrated forward model can be simplified to a system of linearequationsN _(b)(l _(1, . . . ,M))=N ₀∫^(Emax) dES _(wb)(E)exp(−Σμ_(m) l_(m))  (3)

Usually, the number of data measurements (M) is much smaller than thenumber of unknowns (E_(max)). With the assumption of Poissondistribution of the data acquisition, an iterative EM algorithm can bederived to find the optimal estimation of the unknown energy binresponse function S_(wb) (E), as described below.

When estimating the bin response function using low flux dataacquisition, the pileup effect correction P_(b) is assumed to be a knownterm (e.g. constant). So, the model is simplified toN _(b) =N ₀ ∫dE S _(wb)(E)[exp[−Σμ_(m)(E)l _(m)]]  (4)

Let A^(j)(E)=exp[−Σμ_(m)(E)l_(m) ^(j)] represent the attenuatedpathlength for j-th measurement. Thus, for each measurement j, we haveN _(b) ^(j) =N ₀ ∫dE S _(wb)(E)A ^(j)(E)=N ₀Σ_(E) S _(wb)(E)A^(j)(E)  (5)

With M measurements, the data acquisition can be written in the matrixform below

${{N_{0}\begin{pmatrix}{A^{1}(1)} & \ldots & {A^{1}\left( E_{\max} \right)} \\\vdots & \ddots & \vdots \\{A^{M}(1)} & \ldots & {A^{M}\left( E_{\max} \right)}\end{pmatrix}}_{M \times E_{{ma}x}} \cdot \begin{pmatrix}{S_{wb}(1)} \\\vdots \\{S_{wb}\left( E_{\max} \right)}\end{pmatrix}_{E_{\max} \times 1}} = \begin{pmatrix}N_{b}^{1} \\\vdots \\N_{b}^{M}\end{pmatrix}_{M \times 1}$

or A·S_(wb)=N_(b)

By applying the EM iterative algorithm, the S_(wb) can be estimated byS _(wb) ^((k+1)) =S _(wb) ^((k))

((A ^(T)·(N _(b)

(A·S _(wb) ^((k)))))

(A ^(T)·1))  (6)

where

-   -   k: iteration number    -   ·: matrix multiplication    -   : element-wise multiplication    -   : element-wise division    -   1: vector of ones with size of M×1

the updating formula for S_(wb)(E) is given by

$\begin{matrix}{{S_{wb}^{({k + 1})}(E)} = {{S_{wb}^{(k)}(E)}\frac{\sum\limits_{j^{\prime}}{{A^{j^{\prime}}(E)}\frac{N_{b}^{j^{\prime}}}{\sum\limits_{E^{\prime}}{{A^{j^{\prime}}\left( E^{\prime} \right)}{S_{wb}^{(k)}\left( E^{\prime} \right)}}}}}{\sum\limits_{j^{\prime}}\;{A^{j^{\prime}}(E)}}}} & (7)\end{matrix}$

Step 2: Once S_(wb)(E) is estimated from the calibration at each tubevoltage (kVp) setting for each detector pixel, it is saved as a softwarecalibration table on the system. It will be used as an input to furtherestimate the pileup correction terms P_(b) (E, N_(b), N_(tot)) at higherflux scans. Both tables are then used for the material decomposition inobject/patient scans to estimate the basis material path lengths.

The calibration tables are updated from time to time based on thesystem/detector performance variations. This can also be designed as aniterative procedure. If the image quality is not good enough on aquality check phantom, this calibration process is repeated with theupdated calibration tables from the last iteration as the initial guess.

The high level workflow of the above process is demonstrated in FIG. 2.Steps 1) to 4) represent the calibration workflow, and steps 5) to 8)demonstrate how the calibration tables are used in the operational scansof patients/objects to produce spectral images.

First, a series of low flux scans on various material slabs arecollected at each tube kVp setting, which is the peak potential appliedon the X-ray tube. Typical CT systems support a few kVp settings from 70to 140 kVp which generate different energy spectrums from the X-ray tubefor different scan protocol. For a CT scan, both mA and kVp need to bepre-selected before the tube is turned on. Then, the low flux weightedbin response function S_(wb) is estimated and with the estimated S_(wb),the high flux slab scans are used to estimate the additional parametersin the pileup correction term P_(b). With the estimation calibrationtables of S_(wb) and P_(b) for each detector pixel, the quality of thecalibration is checked on a quality phantom, e.g. a uniform waterphantom, or phantom with multiple inserts with uniform known materials.The image quality is assessed with predefined standards, and if it ispassed, the current calibration tables are saved and then used for thefollowing patient/object scans data processing. Otherwise, the proceduregoes through the first three steps again using the last iteration ofS_(wb) and P_(b) as the initial guess. Here, commonly examined standardsare: image CT number accuracy, uniformity, spatial resolution, noise andartifacts. To check the quality of this calibration, these metricsshould all be checked, especially the accuracy and artifacts like ringor bands in the image, which indicate the calibration is not goodenough.

To choose the optimal materials and path lengths for this calibration,one can use the normalized linear attenuation coefficient vs. energycurves (FIG. 3) to choose the ones that are different from each other,e.g. polypropylene, water, aluminum, titanium can be a good group ofcombinations for such calibrations which covers a large range of commonmaterials present in human body.

In order to satisfy the low flux condition through the calibrationmeasurement to minimize the pileup effect in the flow diagram, step 1,one can select to use nτ<x, where x˜0.005-0.01 and n is the pixel countrate with the lowest tube flux setting, and τ is the effective dead timeof the PCD Application Specific Integrated Circuit (ASIC). By satisfyingthis condition, one can calculate the shortest path length of eachselected calibration material, and the rest of path lengths can eitherbe selected by equal spacing in path length or in resulting measurementcount rate.

For calibration of the pileup correction term P_(b) in step 3, the sameslab material and path lengths are used for scans at high mA settings.The calibration data can be grouped for each mA and generate differentcorrection tables for each mA setting (FIG. 4), or include measurementsat all flux ranges (e.g., from low to high mA, from high to low mA, orwith most frequently used values first) to generate a universalcorrection table for a continuous mA setting (FIG. 5).

The calibration measurements should be taken with sufficient statisticsto minimize the influence of the statistical fluctuation. Onenon-limiting example is to use >1000 times more statistics as thetypical integration period for the calibration data sets to minimize thetransferred statistical error in the calibration. Each energy bin b ofthe calibration measurements will be used to update the correspondingS_(wb)(E) and P_(b)(E, N_(b), N_(tot)).

Since one can only do limited number of measurements with a few energybins, the estimation is very ill-conditioned. In this case, a goodinitial guess is crucial for an accurate estimation as it providesadditional constraints for the EM method. One of the design variationsto accommodate non-ideal detectors is to allow a more flexible energywindow for each bin in the initial guess of S_(b), especially with smallvariations in the actual energy threshold setting of the ASIC. Bysetting the low threshold x keV lower, and high threshold y keV higher,the initial S_(b) becomes:S _(b)(E)=∫_(EbL-x) ^(EbH+y) R(E,E′)dE′  (8)=where x, y can be chosen between 5 to 10 keV to allow certain variationsin the ASIC performance, while still providing additional constraintsfor the EM problem.

To capture the spectrum variation across the fan beam after bowtiefilter and detector response variation across different detector pixels,this calibration process is done pixel by pixel with each bowtie/filterconfiguration.

The design described in the present application employs more than twomaterials in the calibration, which provides more sensitivity toconstraint the weighted bin response function estimation problem of thephoton counting detectors.

In addition, the method utilizes a different parameterization for thehigh flux pileup correction terms P_(b) which is now a function of E,N_(b) and N_(tot). The total count term N_(tot) is introduced for abetter approximation of the true pileup phenomena, and can significantlyimprove the model capability at higher flux condition with fewerparameters.

Furthermore, various calibration path length ranges are used atdifferent fan angles to improve the calibration accuracy and efficiency.The slab scans used for the forward model calibration can be selectedbased on the imaging task to generate the best image quality.

Finally, a different scheme is presented to calculate an initial guessof the weighted bin response function by enlarging the energy thresholdwindow, to accommodate non-ideal detector/ASIC performance.

Numerous modifications and variations of the embodiments presentedherein are possible in light of the above teachings. It is therefore tobe understood that within the scope of the claims, the disclosure may bepracticed otherwise than as specifically described herein.

The invention claimed is:
 1. A method for calibrating a response of aphoton counting detector (PCD) for material decomposition in a photoncounting computed tomography (CT) system, the method comprising: (1) ateach pixel of the PCD, performing a plurality of low flux scanscomprising an air scan and scans using slabs of different materials, atan initial current intensity and at each tube voltage setting of anX-ray tube, for each scan, to obtain counts for each energy bin; (2)estimating first parameters, which are dependent on energy, based on thecounts; (3) estimating second parameters which are dependent on totalcounts of all energy bins, based on the first parameters; and (4)performing a material decomposition in object/patient scans to estimatethe basis material based on the first parameters and the secondparameters.
 2. The method according to the claim 1, further comprising:repeating (1) to (3) for different current intensities other than theinitial current intensity and at the same tube voltage and using thesame slabs to obtain the second parameters, respectively, for thedifferent current intensities.
 3. The method according to the claim 2,further comprising: assessing image quality of a projection image byperforming a phantom scan using the first parameters and the secondparameters and if the quality of the projection image satisfiespredefined standards, the first parameters and the second parameters areused for the material decomposition in object/patient scans to estimatethe basis material.
 4. The method according to the claim 1, wherein, theestimating the first parameters, based on the counts, is performed usingexpectation maximization (EM).
 5. The method according to the claim 1,wherein, the low flux scans are defined as occurring when nτ<x, wherex˜0.005-0.01, n is the pixel count rate, and τ is the effective deadtime of the PCD Application Specific Integrated Circuit (ASIC).
 6. Themethod according to the claim 1, wherein, a number N of the differentmaterials used for the slabs is more than one.
 7. The method accordingto the claim 6, wherein, the slabs include polypropylene, water,aluminium, titanium/copper, and k-edge materials.
 8. The methodaccording to the claim 4, wherein an initial guess of the firstparameters is based on detector response function and low and highenergy threshold of each counting bin, respectively.
 9. The methodaccording to claim 1, wherein the calibration is performed pixel bypixel.
 10. The method according to claim 1, wherein the first parametersdepend on a bin response function.
 11. The method according to claim 1,wherein the second parameters are related to pileup correction terms.12. A method for calibrating a response of a photon counting detector(PCD) for material decomposition in a photon counting computedtomography (CT) system, the method comprising: (1) at each pixel,performing a plurality of low flux scans comprising an air scan andscans using slabs of different materials at an initial current intensityand at each tube voltage setting of an X-ray, for each scan, to obtaincounts for each energy bin; (2) estimating first parameters, which aredependent on energy, based on the counts; (3) repeating (1) fordifferent current intensities other than the initial current intensityand at the same tube voltage and using the same slabs, to obtain auniversal table of estimated second parameters which are dependent tototal counts of all energy bins based on the first parameters, in theentire current intensity range; (4) performing a material decompositionin object/patient scans to estimate the basis material based on thefirst and the second parameters.
 13. The method according to the claim12, further comprising: assessing image quality of a projection image byperforming a phantom scan using the first parameters and the secondparameters and if the quality of the projection image satisfiespredefined standards, the first parameters and the second parameters areused for the material decomposition in object/patient scans to estimatethe basis material.
 14. The method according to claim 12 wherein theestimating the first parameters, based on the counts, is performed usingexpectation maximization (EM).
 15. The method according to claim 12,wherein the low flux scans are defined as occurring when nτ<x, wherex˜0.005-0.01 and n is the pixel count rate, and τ is the effective deadtime of the PCD Application Specific Integrated Circuit (ASIC).
 16. Themethod according to claim 12, wherein a number N of the differentmaterials used for the slabs is more than one.
 17. The method accordingto claim 16, wherein the slabs include polypropylene, water, aluminium,titanium/copper, and k-edge materials.
 18. The method according to claim14, wherein an initial guess of the first parameters is based ondetector response function, and low and high energy threshold of eachcounting bin, respectively.
 19. The method according to claim 12,wherein the first parameters depend on a bin response function.
 20. Themethod according to claim 12, wherein the second parameters are relatedto pileup correction terms.